The invention generally relates to protection of power overhead transmission lines and cables where the protected line (or cable) has a transformer (or transformers) tapped between the substations. More particularly, the present invention allows the application of well-known current differential protection principles without measuring the currents at the line terminals of the tapped transformer.
An overhead power transmission line or a cablexe2x80x94referred to as a xe2x80x9clinexe2x80x9d in this documentxe2x80x94can be protected in a variety of ways. The applied protection solution depends on the line voltage level, line configuration, importance of the protected line for the power system as a whole, as well as available communication means and bandwidth for the protection equipment.
Among known protection techniques, the overcurrent protection principle is perhaps the simplest. The principle relies on the magnitude of the current and classifies situations with excessive currents as fault conditions. The principle has many versions including directional and voltage supervision, and definite or current-dependent time-delayed operation. With all these enhancements the overcurrent principle can be applied to certain line configurations only; primarily, on the power distribution level.
The distance protection principle derives an xe2x80x9capparentxe2x80x9d impedance from the voltages and currents measured at the substation and associated with the protected line. The technique for measuring the impedance (either directly or indirectly) insures that the apparent impedancexe2x80x94for any type of faultxe2x80x94is proportional to the actual geometrical distance from the substation to the fault position. Distance relays are capable of locating faults on the line with precision, at least in ideal conditions. Thus, they can protect complex line configurations. In actual situations, however, several factors affect accuracy of distance relays. In order to avoid malfunction of a distance relay on near external faults, the relay reach cannot be set to 100% of the line length but it is set usually at 70-90% depending on the quality of the applied protection equipment.
In order to protect the entire line, several time-delayed overreaching distance zones are usually used. This is known as a stepped distance protection scheme.
By exchanging at least one bit of information, two distance relays installed at both ends of a protected line can be arranged into a so-called pilot scheme. A pilot scheme can be organized either using a permissive or blocking logic and always ensures much more reliable protection comparing with two individual distance relays without any means of communication. There are situations, however, such as simultaneous external and internal faults, where the distance-based pilot scheme may fail to provide reliable protection. Sensitivity of distance protection schemes is another limitation.
The line current differential principle is another protection technique. It compares the currents, typically the current phasors, at both the terminals of the protected line. In the case of an external fault the currents match almost perfectly; while during internal faults, the difference is significant. A line current differential relay creates internally a signal proportional to the difference between the locally measured current and the remote current (the differential current or signal). To cope with line charge currents, and transformation errors of the Current Transformers (CTs) including saturation, the percentage differential principle is used. The differential current is not compared against a threshold but against a specially formed restraining current. Various types of operate/no-operate characteristics can be used to accomplish the percentage current differential characteristic.
There are typically several requirements of the current differential protection principle. First, significant amounts of information must be exchanged over long distances (tens or hundreds of kilometers). Microprocessor-based protective relays exchange their locally measured current phasors in a digital form over a communication channel set up using fiber optic links, microwave channels or some other physical medium.
Second, the microprocessor-based relays at both line terminals must be accurately synchronized in order to measure the line terminal currents at the same time instances. This is accomplished utilizing, for example, well-known Global Positioning System (GPS) as a source of a absolute clock or using some other techniques for self-synchronization of two or more line current differential relays. Such a technique is described in U.S. Pat. No. 4,715,000.
Third, the circuits connected to the protected line such as tapped lines or transformers must be monitored current-wise in order to apply the current differential protection principle. This may create a problem as the tapped connections are meant to provide a cost-effective alternative to actual substations. The taps are made outside the main substations, they are rarely equipped with proper protection means such a Circuit Breakers (CBs) and CTs. Also, high-speed communication from the tap position may be a problem. This either limits application of line current differential relays on tapped lines or makes the tapped line connections economically less attractive.
Presently-known techniques do not adequately address these problems.
The present invention overcomes the problems described above, and achieves additional advantages, by providing for a method or system to apply the line current differential relay to protect tapped lines without the measurements at the tap.
According to exemplary embodiments, the current differential relays are made immune to the load current drawn by the tapped transformer under normal system conditions and to the fault current for faults at the distribution busbar of the tapped transformer. This is accomplished by adding distance supervision to the line current differential scheme. The differential scheme is permitted to trip only if the appropriately set distance relay sees the fault within its operating zone.
The disclosed embodiments use a distance protection element, either as a part of the line current differential integrated protective relay or implemented as a separate relay integrated with the line current differential relay, to supervise the operation of the latter. The distance element is set to detect the faults located on the protected transmission line, and within the tapped transformer, but not the faults located at the distribution busbar of the tapped transformer. As the tapped transformers are of relatively low power, their impedance is relatively high; thus, a reach setting meeting the above requirements is possible. The distance zone is set as an instantaneous zone overreaching the line length with a security margin with respect to the distribution busbar of the tapped transformer. The zone may be set as an overreaching zone because the differential protection element is capable of distinguishing faults on the line or the tapped transformer from the faults outside the line.
Another obstacle is associated with external ground faults. The tapped transformers have typically their primary winding, i.e. the winding coupled to the line, wye-connected with a grounded neutral, and as such, they create a path for the zero sequence current during ground faults. Thus, if an external ground fault occurs, the zero sequence current is fed by the tapped transformer and the current balance monitored by the line differential relay as the differential signal gets upset. The differential scheme would see significant differential current and would malfunction on external ground faults, despite the distance supervision if the external fault is located within the overreaching distance zone.
To overcome this problem, exemplary embodiments of the present invention subtract the zero sequence current from the currents at both line terminals prior to calculating the differential signal. Owing to this, the differential signal is insensitive to the zero sequence component of the current and the current differential scheme becomes stable on external ground faults.
Subtracting the zero sequence current from the differential signal would, however, degrade relay sensitivity by approximately ⅓rd on internal single line to ground faults. To maintain sensitivity, the zero sequence component is subtracted when calculating the restraining signal as well.
Another problem generated by the operation of subtracting the zero sequence component from the differential and restraining signals is that the phase-selective operation of the current differential principle would be lost. Normally, the differential relay is implemented on the per phase basis and responds selectively to various types of faults. Particularly, if a single line to ground fault occurs, the relay can tripxe2x80x94if programmed to do soxe2x80x94the CB only in the defective phase (single pole tripping). This is typically followed by the operation of autoreclosing the breaker after a pre-defined time interval in anticipation of a fault to disappear by itself. This keeps the line in service and ensures certain power transfer between the substations.
When the zero sequence current is removed from the differential current, the differential currents in the healthy phases during an internal single line to ground faults would equal approximately ⅓rd of the fault current. The restraining current would be to too low to prevent relay malfunction in the healthy phases. Consequently, the relay that compensates for the zero sequence current would always trip all three phases. To prevent that the present invention uses both the traditional line current differential characteristic and the modified (zero sequence compensated) line current differential characteristic in parallel, The first one tends to overtrip on external faults. The latter tends to overtrip healthy phases on internal faults. To solve the problem the two differential characteristics supervise each other mutually and the phase-selective operation is regained.
In accordance with the disclosed embodiments, a power line with a tapped transformer can be protected by current differential relays installed exclusively in the substations, without degrading sensitivity and phase-selectivity of the protection on internal faults nor jeopardizing relay stability on external faults.